Planting Tiger Lilies in Ditch

I have a 150 foot draining ditch that is filled with weeds and underbrush. It is impossible to mow and nearly impossible to weed eat. It looks even worse in the winter when the weeds die and the clay dirt shows through. I have decided to line it with Tiger Lilies to make the ditch more attractive as well as protect it from erosion. Should I spray the whole thing with roundup before planting the Lilies or plant the Lilies amongst the weeds and let them naturally choke the weeds out?

Consider Joe Pye weed. It's happy in moist, sunny spots and would look great with your tiger lilies further up the sides. For a plant that would add winter interest, consider the red- or yellow-twigged dogwoods.

Terry I believe there are certain types of Irises that would thrive in the ditch as well.
Doug you are so right about the JPW - I see it on my way to work on the country roads - it loves the ditched - they look so healthy there. I have a tall one in my back yard - It is in a sunny spot and is 7' -- the road side ones only get 3-4'.

It looks like the Joe Pye weed prefers moist soil. My drainage ditch often becomes bone dry when there is little rain or when it is extremely hot so it may not do well for me. I like the idea of planting the tiger lilies along the outer edge of the ditch but I really need to get the weeds in the ditch under control. Is there a fast spreading ground cover that I can use that would tolerate dry conditions and getting wet occasionally? I'm adding some images to show how bad the ditch looks.

Joe Pye likes moist soil but doesn't need to have it. Wouldn't a ground cover not just be wet but submerged? I believe Carol is referring to the flag irises. The blue is native, the yellow is not, and is far less well-behaved. It would certainly choke out any weeds that are there, but you may have trouble keeping it in control.

Up here in RI - everyone calls it JPW - it 's blossoms are pink and resssenble that of Seddum. The butterflies seem to love it. Um - You could plant infront of the ditch, to hide at least some of the weeds. - A few small bushes, ornimental trees, Lillies, Hosta, (a varity that like a sunny location).
OR - simply spray the ditch with vinager -- safer than weed kill -